Cyclohexane
is a cycloalkane with the molecular formula C6H12.
Cyclohexane is used as a nonpolar solvent for the
chemical industry, and also as a raw material for
the industrial production of adipic acid and caprolactam,
both of which are intermediates used in the production
of nylon. On an industrial scale, cyclohexane is produced
by reacting benzene with hydrogen. Due to its unique
chemical and conformational properties, cyclohexane
is also used in labs in analysis and as a standard.

Chemical
conformation

The
6 vertexed ring does not conform to the shape of a
perfect hexagon. The conformation of a flat 2D planar
hexagon has considerable angle strain due to the fact
that its bonds are not 109.5 degrees; the torsional
strain would also be considerable due to all eclipsed
bonds. Therefore, to reduce torsional strain, cyclohexane
adopts a three-dimensional structure known as the
chair conformation. The new conformation puts the
carbons at an angle of 109.5°. Half of the hydrogens
are in the plane of the ring (equatorial) while
the other half are perpendicular to the plane (axial).
This conformation allows for the most stable structure
of cyclohexane. Another conformation of cyclohexane
exists, known as boat conformation, but it interconverts
to the slightly more stable chair formation. If cyclohexane
is mono-substituted with a large substituent, then
the substituent will most likely be found attached
in an equatorial position, as this is the slightly
more stable conformation.

Cyclohexane
has the lowest angle and torsional strain of all the
cycloalkanes, as a result cyclohexane has been deemed
a 0 in total ring strain, a combination of angle and
torsional strain. This also makes cyclohexane the
most stable of the cycloalkanes and therefore will
produce the least amount of heat when burned compared
to the other cycloalkanes.

A
cyclohexane molecule in chair conformation.
Hydrogen atoms in axial positions are shown in
red, while those in equatorial positions are in
blue.

Reactions with cyclohexane

Pure
cyclohexane in itself is rather unreactive, being
a non-polar, hydrophobic hydrocarbon. It can react
with very strong acids such as the superacid system
HF + SbF5 which will cause forced protonation
and "hydrocarbon cracking". Substituted cyclohexanes,
however, may be reactive under a variety of conditions,
many of which are important to organic chemistry.
Cyclohexane is highly flammable.

Cyclohexane
derivatives

The
specific arrangement of functional groups in cyclohexane
derivatives, and indeed in most cycloalkane molecules,
is extremely important in chemical reactions, especially
reactions involving nucleophiles. Substituents on
the ring must be in the axial formation to react with
other molecules. For example, the reaction of bromocyclohexane
and a common nucleophile, a hydroxide anion , would
result in cyclohexene.

This
reaction, commonly known as an elimination reaction
or dehalogenation (specifically E2), requires that
the bromine substituent be in the axial formation,
opposing another axial H atom to react. Assuming that
the bromocyclohexane was in the appropriate formation
to react, the E2 reaction would commence as such:

The
electron pair bond between the C-Br moves to the
Br, forming Br− and setting it free
from cyclohexane

The
nucleophile (-OH) gives an electron pair to the
adjacent axial H, setting H free and bonding to
it to create H2O

The
electron pair bond between the adjacent axial H
moves to the bond between the two C-C making it
C=C

Note:
All three steps happen simultaneously, characteristic
of all E2 reactions.

The
reaction above will generate mostly E2 reactions and
as a result the product will be mostly (~70%) cyclohexene.
However, the percentage varies with conditions, and
generally, two different reactions (E2 and Sn2) compete.
In the above reaction, an Sn2 reaction would substitute
the bromine for a hydroxyl (OH-) group
instead, but once again, the Br must be in axial to
react. Once the SN2 substitution is complete,
the newly substituted OH group would flip back to
the more stable equatorial position quickly (~1 millisecond).Uses

Commercially
most of cyclohexane produced is converted into cyclohexanone-cyclohexanol
mixture by catalytic oxidation. KA oil is then used
as a raw material for adipic acid and caprolactam.
Practically, if the cyclohexanol content of KA oil
is higher than cyclohexanone, it is more likely(economical)
to be converted into adipic acid, and the reverse
case, caprolactam production is more likely. Such
ratio in KA oil can be controlled by selecting suitable
oxidation catalyts. Some of cyclohexane is used as
an organic solvent.

Cyclohexane
in research

Although
much is already known about this cyclic hydrocarbon,
research is still being done on cyclohexane and benzene mixtures and solid phase cyclohexane to determine
hydrogen yields of the mix when irradiated at −195
°C.

History

Unlike
compounds like benzene, cyclohexane cannot easily
be obtained from natural resources such as coal. Towards
the end of the nineteenth century early chemical investigators
had to depend on organic synthesis. It took them 30
years to flesh out the details[1]. In 1867 Marcellin Berthelot reduced benzene with hydroiodic
acid at elevated temperatures. He incorrectly identified
the reaction product as n-hexane not only because
of the convenient match in boiling point (69°C) but
also because he did not believe benzene was a cyclic
molecule (like his contemporary August Kekule) but
rather some sort of association of acetylene. In 1870
one of his sceptics Adolf von Baeyer repeated the
reaction and pronounced the same reaction product
hexahydrobenzene and in 1890 Vladimir Markovnikov
believed he was able to distill the same compound
from Caucasus petroleum calling his concoction hexanaphtene

In
1894 Baeyer synthesized cyclohexane starting with
a Dieckmann condensation of pimelic acid followed
by multiple reductions. and in the same year E. Haworth
and W.H. Perkin Jr. (1860 - 1929) did the same in
a Wurtz reaction of 1,6-dibromohexane.Surprisingly
their cyclohexanes boiled higher by 10°C than either
hexahydrobenzene or hexanaphtene but this riddle was
solved in 1895 by Markovnikov, N.M. Kishner and Nikolay
Zelinsky when they re-diagnosed hexahydrobenzene and
hexanaphtene as methylcyclopentane, the result of
an unexpected rearrangement reaction.

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